1. Field of the Invention
The present invention relates to an electrically-operated injection molding machine, and more particularly, to a device for determining the load exerted on a movable part of an injection molding machine.
2. Description of Related Art
In injection molding machines, a method has often been adopted that determines abnormality of a movable part based on the load applied thereto. Especially in electrically-operated injection molding machines, in order to detect abnormality of the movable part, etc., the load exerted on a moving-part driving motor is detected based on the motor current or the like, without using a special hardware sensor such as a load sensor for detecting the load exerted on the movable part.
For example, there has been known an invention in which the load exerted on an ejector is estimated by means of a load estimating observer incorporated into a controller for an ejector driving motor, and an abnormal action is detected based on the estimated load, thus preventing an ejector rod from colliding with a member in a mold due to an error in setting an amount of projection of the ejector rod, so as to prevent in advance damages of the mold member and the ejector mechanism. Apart from estimating the load by means of the load estimating observer, the just-mentioned invention also adopts a method for detecting the load based on a commanded motor current or an actual motor current in view of the fact that the motor driving current varies in proportion to the load exerted on the motor (refer to JP 10-119107A).
Also known is a method for sampling the estimated disturbance torque at intervals of a predetermined period, which is similarly estimated by means of the observer, and for detecting an abnormal load when an average value of sampled torques exceeds an allowable value (refer to JP 2001-38775A). It is also known to detect the driving current for a knock-pin driving motor during the process of pushing a molded product out of the mold by means of knock pins, and determine an occurrence of abnormality when a driving current pattern is deviated out of an allowable range defined by a reference pattern (refer to JP 2002-18924A).
There has also been known an invention in which the load exerted on an ejector driving motor is estimated by use of a disturbance estimating observer or the like, the mold release force with which a molded product is released from the mold is determined based on the estimated load, and the state and quality of the molded product is detected based on the magnitude of the mold release force (JP 2002-144383A).
As mentioned above, the electrically-operated injection molding machine is designed to detect the load exerted on the moving-part driving motor based on the motor driving current value or by means of the observer incorporated into the motor control circuit, and on the basis of the detected load, determine the quality of molded products as well as occurrences of abnormality.
In the electrically-operated injection molding machine, the detection of load exerted on the movable part being made based on the driving current of the moving-part driving motor or by means of the observer is advantageous in that it results in a low-price arrangement which does not require any hardware such as a torque sensor for load detection.
When the electrically-operated injection molding machine is continuously operated, however, the motor temperature increases with elapse of time. Such change in motor temperature results in a change in the driving current and in the estimated torque detected by the observer, even when the motor load remains unchanged.
FIG. 4 shows a relationship between motor temperature and load detected by the disturbance load observer, which is observed when the actual motor load is constant. With the increase in motor temperature, the load detected by the disturbance load observer increases as illustrated, and deviates from the actually applied load.